The present invention relates to a torque detecting apparatus for detecting a steering torque applied to a steering member for steering in an electric power steering apparatus or the like.
In an electric power steering apparatus in which a motor for steering assist is driven in accordance with a rotation operation of a steering member such as a steering wheel so that the torque of the motor is transmitted to a steering mechanism and thereby assists the steering, the steering torque need be detected that is applied to the steering member for the driving control of the motor for steering assist. In the prior art, for the detection of the steering torque, a torque detecting apparatus is used that is arranged in the middle of a steering shaft that links the steering member and the steering mechanism.
In this torque detecting apparatus, a steering shaft serving as detection target is divided into a first shaft and a second shaft coaxially connected with each other through a torsion bar having a small diameter and serving as a torsion spring. When a steering torque is applied to the steering shaft by a rotation operation of the steering member, a relative angular displacement arises between the first and the second shafts in association with torsion in the torsion bar. Then, the steering torque is detected on the basis of the relative angular displacement.
The detection of the relative angular displacement between the first and the second shafts is implemented by various kinds of means in the prior art. In an example, a torque detecting apparatus comprises: a cylindrical permanent magnet that rotates integrally with the first shaft; and yoke rings that rotate integrally with the second shaft, so that a change is utilized that arises in a magnetic circuit between the cylindrical permanent magnet and the yoke rings (see, for example, Japanese Patent Application Laid Open No. 2003-149062).
In the yoke rings that rotate integrally with the second shaft, a plurality of magnetic pole claws extending in an axial direction on one side of each yoke body having a ring shape are arranged at equal intervals in a circumferential direction. Then, a pair of the yoke rings in which magnetic pole claws of these rings are arranged alternately in the circumferential direction are fixed to the second shaft. Further, the cylindrical permanent magnet that rotates integrally with the first shaft is a multi-pole magnet provided, in the circumferential direction, with magnetic pole groups of the same number as the magnetic pole claws groups of the yoke rings. The cylindrical permanent magnet is fixed to the first shaft in such a manner that in a neutral state where no relative angular displacement arises between the first and the second shafts, the magnetic pole claws of the yoke rings align on the boundaries between the N and S poles.
On the outside of the two yoke rings, flux concentrating rings for collecting the magnetic flux generated in these yoke rings are arranged such as to closely oppose respective yoke bodies. These flux concentrating rings have flux concentrating sections extending from each ring and opposing to each other with a predetermined gap in between. Then, a magnetic sensor composed of a magnetic detector element such as a Hall element is arranged in the gap between these flux concentrating sections.
In this configuration, when a steering torque is applied to the first shaft and the second shaft so that a relative angular displacement arises between these shafts, the positional relation in the circumferential direction between the magnetic pole claws of the two yoke rings and the magnetic poles of the cylindrical permanent magnet varies in the opposite direction with each other. Then, the change in the magnetic flux in the respective yoke rings in association with the positional change causes a change in the leakage flux in the gap between the flux concentrating sections of the flux concentrating rings. Accordingly, when a change in the output of the magnetic sensor in association with the above-mentioned change is extracted, a torque (steering torque) is detected that is applied to the first and the second shafts.